1. Field of the Invention
This present invention relates generally to the field of telecommunications and, more specifically, to arrangements for providing redundancy, expanded switching capacity and fault isolation in an expandable telecommunications system.
2. Discussion of the Related Art
In most telecommunications systems, it is highly desirable, if not necessary, to provide some measure of redundancy with respect to some or all parts of the system. Such redundancy is needed to ensure that the system will continue to operate, preferably with no degradation of performance, in the event that a particular subsystem or component malfunctions or must be taken out of service.
Redundancy may be provided in a number of ways, the choice of which typically depends upon the cost, the importance of a particular device to overall system performance, the degree of difficulty of replacing the device and other factors. One type of redundancy, often referred to as "one-to-one" redundancy, is based on the concept that each "active" device is matched or paired with an identical "standby" device. In the event that the active device fails, a "cutover" occurs in which the standby device functionally replaces the failed device.
A major disadvantage of conventional one-to-one redundancy is that, at the time of cutover, the original active device and the standby device are in non-identical operating states. Any difference between those operating states will generally result in interruption or loss of service, which may be manifested in the form of dropped calls, calls which fail to connect, and the like. Such degraded performance is generally unacceptable to service providers and customers, at least to the extent that it affects more than a very small percentage of the total call traffic for a very short period of time.
One alternative to the one-to-one redundancy approach is to provide a limited number of standby components which are available to replace some, but not all, of a larger number of active components. This approach is often referred to as "n+1" redundancy where n refers to the number of active components. While the n+1 approach is less costly than the one-to-one approach, there is correspondingly less protection against degraded system performance. That is, in the event that several active devices fail at about the same time and that number exceeds the number of available standby devices, then some loss of service or degradation of performance will occur.
Another disadvantage of the n+1 approach is that because it is not known in advance which one of several active devices may fail, there is no way to maintain a particular standby device in an operating state which matches that of the device which is to be replaced. Consequently, in the n+1 redundancy approach, there is a high likelihood that service will be disrupted during cutover from a failed device to a standby device.